Stoichiometry

Stoichiometry

Equations on a Macroscopic Scale

2 2 33 2N H NH

32 2

On a microscopic scale. this means

2 molecules1 molecule 3 molecules

of NH of N of H

32 2

Multiplying the equation by Avogadro's no. converts

the quantities to mols

2 mols 1 mol 3 mols

of NH of N of H

Mole Ratios from a Balanced Chemical Equation

2 2 33 2N H NH

222

2

1 mol N1 mol N : 3 mols or

3mols HH

232

3

1 mol N1 mol N : 2 mols or

2mols NH

NH

232

3

3 mol H3 mols H : 2 mols or

2 mols NH

NH

Mass Ratios from a Balanced Chem Equation

2 2 33 2N H NH

22 2

2

28 g N28 g N : 3 2 g

6 g H

H

22 3

3

28 g N28 g N : 2 17 g

34 g NH

NH

22 3

3

6 g H3 2 g : 2 17 g

34 g H NH

NH

2 2 3

1 mol 3 mol 2 mol

28 g

3 2 g

3

2 17

2

g

N H NH

28 g 3 mol

1 mol 3 2 g

3 mol 2 17 g

3 2 g 2 mol

28 g 2 mol

1 mol 2 34 g

Exercise

4.7/148For the ffg rxns, write the ratios that can be established among molar amounts of the species.

2 2 2 2

4 2 4 10

23 2

a. 2 2

b. 5

c. 2 2 3MnO

H O H O O

P O P O

KClO KCl O

Calculating Amounts of Reactants and Products

1. Balance the equation for the reaction.2. Use the balanced equation to set up the

appropriate mole or mass ratios.3. Use the appropriate mole ratios to calculate

the number of moles of the desired reactant or product.

4. Convert from moles to grams if required by the problem.

Example Exercises

1. Cl2 is used by textile manufacturers to bleach cloth. Excess Cl2 is destroyed by its rxn with sodium thiosulfate, as follows:

1. How many moles of sodium thiosulfate are needed to react with 0.12 mol of Cl2 ?

2. How many moles of HCl can form from 0.12 mol of Cl2 ?

3. How many moles of water are required to react with 0.12 mol of Cl2?

4. How many moles of water react if 0.24 mol of HCl is formed?

2 2 2 23 4 aq g l aq aqNa S O Cl H O NaHSO HCl

4.10/148For the rxn:

3.7 moles of H2 are mixed with excess Cl2. How many moles of HCl can be formed?

4.12/148In petroleum refining, hydrocarbons are often manipulated by reacting them with H2 (g). If hexene , C6H12, is reacted with H2 to form hexane, , C6H14, how many moles of hydrogen gas are needed to react with 453 moles of hexene?

2 2 2H Cl HCl

Given the rxn:

a. How many g of N2H4 are required to react with 25.49 g of N2O4?

b.How many g of N2 will be formed from the rxn of 89.7 g N2H4 assuming enough N2O4 is available?

2 4 2 4 2 22 3 4N O l N H l N g H O g

• Given the equation:

a. How many moles of HCl are required to react with 31.8 g of MnO2?

b. How many g of MnO2 must react for 0.56

mol of Cl2 to be formed?

2 2 2 24HCl aq +MnO s →2H O l +MnCl aq +Cl g

Coke is an impure form of carbon that is often used in the industrial production of metals from their oxides. If a sample of coke is 95 % carbon by mass, determine the mass of coke needed to react completely with 1 metric ton of copper (1) oxide. The other product is carbon dioxide.

When heated, mercury (11) oxide decomposes to oxygen gas and mercury vapor. a. How many g of mercury (11) oxide must be

decomposed to form 0.567 mol of oxygen?b. how many g of mercury will be

subsequently formed?

Limiting Reactant

• or limiting reagent• The material which is present in smaller amount

according to the balanced chemical equation will control the extent of the reaction.

• The other material is said to be in excess. • The limiting reactant will use up all the reactants

and produce the quantity of product that conserves mass.

• This stoichiometric amount is called the theoretical yield.

Solving Stoichiometry Problems

• If quantities of more than one reactant are given, determine which reactant is limiting by finding the fraction of a recipe of each reactant present.

• The qty of the limiting reactant determines the amount of product formed and the amounts of other reactants that react.

Exercises

1. Given the equation:

How many g of AgCl will be formed if a solution that contains 41.6 g AgNO3 is mixed with a solution that contains 35.4 g of BaCl2?

3 2 3 22AgNO aq +BaCl aq →2AgCl s +Ba NO aq

The rxnTakes place at very high temperatures. If 42.7 g

of Cr2O3 and 9.8 g of Al are mixed and reacted until one reactant is used up, which reactant will be left over? How much of it will be left over?

2 3 2 32 2Cr O s Al l Cr l Al O l

Theoretical, Actual and % Yields

• Amount of product calculated to be formed by the reaction is the theoritical yield.

• Actual yield are usually less that the theoretical yielda. Reactant is impureb. Some products are lost mechanically.c. There may be side reactions

• actual yield

= 100theoretical yiel

% yied

ld

Given the equation:

a. What is the theoretical yield of Fe2O3 from the reaction of 4.8 g Fe with excess O2?

b. If 6.76 g of Fe2O3 are actually obtained from 4.86 g Fe, what is the % yield?

c. Under other conditions, the % yield is 75.6 %, how many g of Fe2O3 are formed from 4.86 g of Fe under these conditions?

2 2 34 3 heatFe s O g Fe O s

If 2.85 g of thallium chloride is formed by the reaction of 2.43 g of thallium metal with an excess of chlorine gas,a. What is the % by mass, of thallium in the

thallium chloride?b. % chlorine?

Ratios From a Balanced Chemical Equation

Example: For the rxn: we can write the ffg sets of mole ratios

2 3 22 3 3 4Fe O C CO Fe

2 3

2 3 2

2 3

2 mols Fe O : 3 mols C

2 mols Fe O : 3 mols C

2 mols Fe O : 4 mols C Fe

O2

2

3 mols C : 3 mols C

3 mols C : 4 mols Fe

3 mols C : 4 mols Fe

O

O